Re: [PATCH] x86: Add thresholds for "rep movsb/stosb" to tunables

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, May 29, 2020 at 6:13 AM Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 5/23/20 12:37 AM, H.J. Lu via Libc-alpha wrote:
> > There is no single threshold value which is good for all workloads.
> > I don't think we should change REP_STOSB_THRESHOLD to 1MB.
> > On the other hand, the fixed threshold isn't flexible.  Please try this
> > patch to see if you can set the threshold for your specific workload.
>
> My request here is that the manual include a documentation of what the
> minimums are for the tunable. Even an example reference of the minimum
> value would be useful for the tunable e.g. On AVX512 systems this value
> is X, on AVX systems this value is Y, on all other systems Z.
>

The logic of thresholds are:

 /* NB: The REP MOVSB threshold must be greater than VEC_SIZE * 8.  */
  unsigned int minimum_rep_movsb_threshold;
  /* NB: The default REP MOVSB threshold is 2048 * (VEC_SIZE / 16).  */
  unsigned int rep_movsb_threshold;
  if (CPU_FEATURES_ARCH_P (cpu_features, AVX512F_Usable)
      && !CPU_FEATURES_ARCH_P (cpu_features, Prefer_No_AVX512))
    {
      rep_movsb_threshold = 2048 * (64 / 16);
      minimum_rep_movsb_threshold = 64 * 8;
    }
  else if (CPU_FEATURES_ARCH_P (cpu_features,
AVX_Fast_Unaligned_Load))
    {
      rep_movsb_threshold = 2048 * (32 / 16);
      minimum_rep_movsb_threshold = 32 * 8;
    }
  else
    {
      rep_movsb_threshold = 2048 * (16 / 16);
      minimum_rep_movsb_threshold = 16 * 8;
    }
  if (cpu_features->rep_movsb_threshold > minimum_rep_movsb_threshold)
    __x86_rep_movsb_threshold = cpu_features->rep_movsb_threshold;
  else
    __x86_rep_movsb_threshold = rep_movsb_threshold;

We can't simply say AVX512 machines will use ZMM and AVX machines
will use YMM.  It depends on other factors which are invisible to users.
Can you suggest some paragraph for libc manual?

Thanks.

-- 
H.J.